1. Field of the Invention
The present invention relates generally to techniques which are used during the fabrication of semiconductor devices. More specifically, the invention relates to a technique which uses multiple mappings of critical dimensions of selected features formed on a wafer during the process of forming integrated circuits, to enable a new etch process to be qualified in an efficient manner and with particular regard to mitigating effects which are encountered during the various steps which are carried during the constructive processes.
2. Description of the Related Art
When a new etch process is introduced, it is difficult to predict the exact results which will be achieved using the same. Accordingly, it is necessary to conduct tests which will reveal the degree to which a positive or negative bias, or other effects such as incomplete etching, overetching including undercutting and/or resist lifting etc., have on the type of etching process being used.
In the case of wet etching of silicon, it is common to use a solution of nitric and hydrofluoric acids. Depending on the ratio of the acids (viz., the recipe) the reaction can be rendered exothermic wherein the heat which is generated tends to speed up the etching process. In order to attenuate the run-away reaction being initiated it is necessary to exert some degree of control and add some modulating agent such as acetic acid or the like. A further factor influencing the process is the crystal orientation which can be used to control the shape of the bottom of a trench which is being created.
Hydrofluoric acid is widely used to etch silicon dioxide. However, in this instance also the recipe is of importance. For example, hydrofluoric acid is usually mixed with water and is buffered with ammonium fluoride to attenuate the generation of hydrogen ions which tend to accelerate the rate of etching. In the case of aluminum problems can be experienced in the event that tiny bubbles of hydrogen gas are produced. These bubbles tend to adhere to the surfaces which are being etched and caused localized slowing of the process. The lack of uniformity of material removal and/or and unexpectedly low etching degree is thus experienced.
In the case that etching of silicon nitride is required, a problem is encountered in that it is necessary to use hot phosphoric acid and, apart from the problems that handling this type of material at temperatures of about 180.degree. C. also tends to cause thermal damage to the resist which is being used as the etch mask.
An example of wet etching which uses a combination of potassium hydroxide, ethylene glycol and water is disclosed in U.S. Pat. No. 3,909,325 issued on Sep. 30, 1975 to Church et al. This reference is hereby incorporated herein by reference.
Dry etching on the other hand. can include plasma, ion milling and reactive ion etching (RIE). As is well known, in the case of plasma etching, gases are introduced at controlled rates while the pressure in the chamber is reduced. The plasma is activated through the application of power at a selected radio frequency or frequencies. However, the effect of temperature, especially the surface temperature of the substrate being etched, must be carefully monitored along with other factors such as the flow rate of the reactants, the ion density, chamber pressure, etc. Further, the effects of plasma/radiation damage must be taken into consideration.
An example of plasma etching is given in U.S. Pat. No. 4,115,184 which was issued on Sep. 19, 1978 in the name of Poulsen. The content of this document is hereby incorporated herein by reference.
However, in the event that a new etch process is being introduced to a production line, it is necessary to taken a large number of factors into consideration depending on the type of etching and the substrate involved. Therefore, the random type of approach of determining which factors require adjustment and the direction of adjustment that is necessary tends to require an excessive amount of experimentation.
Accordingly, there exists a need for a reliable technique by which a new etch process can be introduced and qualified in a manner which identifies the problems that need to be addressed in order to achieve production set-up quickly and relatively inexpensively.